There are many applications in which an optical transmitter, such as a laser diode (LD) or a light-emitting diode (LED), is driven by a pulsed electrical current to alternately switch the light source on and off, and to produce optical pulses.
Typically, it is desired that the optical pulses be of a substantially rectangular shape and follow the current pulses as closely as possible. There is, however, a time delay associated with switching the optical transmitter on or off, e.g. a time delay between an on-off transition in the drive current of the transmitter and an associated on-off transition in the light output thereof. Similarly, there is a time delay between an off-on transition in the drive current of the transmitter and an associated off-on transition in the light output thereof. This time delay can be seen as a consequence of a relatively large capacitance of a typical laser diode or light-emitting diode, which requires a certain time to be charged or discharged by the diode drive current, and/or a relatively large minority carrier life time in the active region of the diode.
The time delays associated with the laser or LED turn-on and turn-off are detrimental for higher-speed applications, e.g. when the transmitter is modulated at frequencies exceeding about 10 to 100 MHz, depending on the transmitter design. To mitigate this problem, prior-art drivers for pulsed laser diodes and LEDs often include pulse shaping circuitry that pre-shapes electrical pulses that drive the transmitters by adding a short positive peaking pulse at a leading edge of the pulses for overshooting the steady-state drive current in the diode's “on” state to kick-start the transmitter, and/or for adding a short negative or inverted peaking pulse at a trailing edge of the driving pulse to more quickly extinguish the transmitter.
Various types of such pulse shaping circuitry that adds peaking to the drive current pulses for laser diodes and LEDs have been disclosed in the past. For example, U.S. Pat. No. 4,818,896, issued to V. Cavanna, discloses a driver for an electro-optical transducer, such as an LED, which shapes current pulses so as to contain “spikes” during turn-on and turn-off in order to quickly charge and discharge the junction and stray capacitances of the LED. The driver includes a “peaking” circuit based on a differential amplifier supplying a “peaking” current to another differential amplifier functioning as a current switch supplying the drive current to the LED. A capacitor couples the amplifiers to conduct the additional current to the LED when it is initially turned on and charges the cathode of the LED. Degeneration resistors interconnecting emitters of the switching transistors couple the amplifiers to current sources and allow the amplifiers to be driven by emitter-coupled logic (ECL) gates.
Other examples of LD or LED drivers incorporating either transistor-based or purely passive peaking circuits have been disclosed. Often, the peaking circuits incorporate a differentiating RC circuit that differentiate an input voltage data pulse that is used to drive the transmitter to form positive and negative peaking voltage pulses at the rising and falling edges of the input data pulse, respectively, and then superimpose the peaking pulses to the transmitter drive current to form positive and negative current spikes at the turn-on and turn-off transitions. Examples of such drivers are disclosed in U.S. Pat. No. 5,343,323 in the names of Lynn, et al, U.S. Pat. No. 5,115,147 in the names of Kusano, et al, and U.S. Pat. No. 6,724,376 issued to Sakura, et al.
U.S. Pat. No. 6,049,175 issued to Forsberg teaches another passive arrangement for peaking the drive current of a light emitting device, e.g. an LED, upon switching it on and off. The peaking arrangement includes a passive peaking network, e.g. formed by a capacitor, a resistor, and an inductor connected in series, which is arranged in parallel with the light emitting device.
U.S. Pat. No. 6,901,091 discloses a driver for a directly-modulated semiconductor laser incorporating a peaking circuit, which comprises a transistor whose emitter terminal is connected to the semiconductor laser across a resistance and whose base current is determined by a base-emitter control voltage that exhibits positive and negative peaks according to the leading and trailing edges of the current through the semiconductor laser, whereby a constant current is generated at the transistor by means of a current mirror and modulated in correspondence with the base-emitter control voltage.
Although the aforementioned inventions appear to perform their intended functions, they provide solutions wherein the shape of the pulses produced by the laser or LED drive circuit is determined by the circuit elements and cannot be easily adjusted once a corresponding circuit board or an IC chip has been fabricated. Such drivers may therefore require extensive tuning in manufacturing. Furthermore, since different lasers and LED may have different electrical characteristics and different rise/fall characteristics associated with the laser/LED turn-on and turn-off, different driver boards and IC chips may be required to drive different output devices. Moreover, if electrical characteristics of the circuit components or the laser/LED change, new driver boards and/or new IC chips may need to be fabricated during the lifetime of the product to maintain the performance of the transmitter.
In a further disadvantage, many of the prior art pulse-shaping circuits do not discriminate between rising and falling edges of the driving electrical pulse; i.e. a peaking pulse added by the circuit to the rising edge is also added in the same magnitude to the falling edge of the driving pulse. However, some types of the optical transmitters, i.e. many types of semiconductor lasers, may require a larger peaking pulse at a rising edge of the drive current pulse than at the falling edge thereof, since lasers often require a larger turn-on pulse than a turn-off pulse.